Decorative articles often utilize a polyurethane top coat to cover or protect a base substrate and any indicia or decorative features located on the substrate. The articles may include various substrates upon which printing or profiled graphics have been applied. The polyurethane material is generally clear or optically transparent and functions as a lens in order to display the substrate or any indicia applied thereon. The decorative articles have a wide variety of uses in both interior and exterior applications. For example, model names for automobiles are often displayed in a decorative manner on an exterior surface of the vehicle. The polyurethane covers and protects the indicia displayed on the model name plate.
Polyurethanes are generally applied onto articles or substrates as a solution or flowable reaction mass which is then cured to form a lens. The flow characteristics of the uncured polyurethane are important in developing a polyurethane coating at a sufficient and desired thickness upon curing.
Some polyurethanes, upon curing, are rigid, non-flexible polymers. The rigidity of the article is often due to the chemical structure of the components used in the formulation of the polyurethane. Rigid polyurethanes are susceptible to abrasion and scratching. Additionally, the materials do not demonstrate an appropriate level of self-healing. Self-healing describes the ability of the polyurethane to return to its original shape or appearance after being deformed, such as by scratching.
Polyurethanes are applied onto various types of substrates. However, some specific substrates are not suitable for use with conventional polyurethanes. Some substrates are susceptible to absorbing an amount of moisture which can react with the polyurethane to form bubbles in the cured polyurethane. The formation of bubbles in the polyurethane is termed outgassing. For polyurethanes containing an isocyanate, outgassing may also occur when certain porous substrates, for example polyvinyl chloride, are backed with adhesives containing carboxyl groups. The porous substrate permits the reaction of the isocyanate in the polyurethane with carboxyl groups in the adhesive. This reaction results in the release of carbon dioxide which generally becomes trapped in the polyurethane. The occurrence of outgassing in a cured polyurethane is aesthetically undesirable.
Conventional two-component polyurethanes are typically either solvent based or utilize polyethers as the primary hydroxyl source. The solvent based polyurethanes are generally not viscous enough to provide a desired thickness to the polyurethane lens. Additionally, solvent based polyurethanes, when applied as top coats, do not permit the complete evaporation of the solvent from the polyurethane. The incomplete evaporation can produce striations, parting lines, or bubbles in the polyurethane and therefore result in an aesthetically unacceptable top coat. Polyether based polyurethane mixtures can provide suitable flexible properties. However, they tend to degrade and yellow when exposed to sunlight. Thus, existing polyurethane compositions, and methods for producing them, often result in undesirable finished properties or are unsuitable for exterior applications.
There is a need to provide a polyurethane that is flexible, durable, and weather resistant. Additionally, such a polyurethane should be applicable to various substrates without experiencing a substantial amount of outgassing. A polyurethane having the noted properties would be suitable for use as a protective coating on various substrates for both interior and/or exterior applications.